1. Field of the Invention
The present invention relates to a thermal head suitable for use as a print head in various recording instruments such as printers.
2. Description of the Related Art
Conventional recording systems such as page printers, word processors, facsimiles and the like have all used thermal print heads. FIG. 3 shows a thermal head, the detail of which is further shown in FIG. 5 by fragmentary cross-section. FIG. 6(b) is another cross-sectional view, enlarged in scale, of the thermal head, taken along a line B--B in FIG. 5. In these figures, the edge of a substrate 1 of alumina ceramic (dielectric) is formed with a glaze layer 2 which is a heat accumulating layer. For each dot, a resistance layer 3, individual lead electrode 4, common electrode 5 and protective film 6 are formed on the substrate 1 over the glaze layer 2 in the order as just described. If a signal is selectively applied to an individual lead electrode 4 through the common electrode 5, the resistance layer 3 located between these electrodes will be heated and so cause the transfer of the ink of a thermal transfer ribbon 8 onto a recording sheet.
Such thermal heads are generally used with a resinous ink such that the thermal transfer ribbon 8 can be used to print or any recording sheet even if it has a rough printing surface. In order to prevent the resinous ink from being re-fused to the recording sheet, therefore, the thermal transfer ribbon 8 must be separated from the recording sheet as rapidly as possible after the transfer of ink has been completed. As shown in FIG. 6(b), thus, the chamfered edge of the conventional thermal head includes a printing portion 100 (above the resistance layer 3 between the individual electrode 4 and the common electrode 5).
More particularly, after the glaze layer 2, resistance layer 3, individual lead electrode 4, common electrode 5 and protective film 6 have been formed sequentially on the substance 1 as shown in FIG. 6(a), the substrate 1 will be cut by means of a diamond disc as shown in FIG. 6(b). Such a cutting operation comprises two separate steps, that is, one step for cutting the glaze layer 2 and protective film 6 which are of glass material by the use of a cutting edge having a fine particle size and another step for cutting the substrate 1 of alumina ceramic by the use of a cutting edge having a rough particle size.
In the conventional thermal head as shown in FIG. 6(b), however, so-called "chipping" may occur which results in that the protective film 6 is separated from the glaze layer 2 by an impact which is produced by the diamond cutting disc. This is because the surface of the glaze layer 2 is very smooth and makes less contact with the protective film 6 which is made of a material similar to the glass material such as tantalum pentoxide and the like. The chipping causes the moisture barrier properties to be lowered so that the common electrode 5 may become corroded, leading to various problems such as failure in conductivity amongst other things.
In order to overcome the problems, the prior art has proposed a thermal head which comprises a protecting and braking layer (7 in FIG. 6(c)) or a bonding layer formed outside the common electrode 5 (or at a position adjacent to the edge of the thermal head), as disclosed in Japanese Patent Laid-Open No. Sho 62-208961, 62-124962 or 63-267564. Such a layer serves to improve the adhesion between the glaze layer 2 and the protective film 6. However, the provision of the protecting and braking layer 7 as shown in FIG. 6(c) increases the spacing between the printing portion 100 and the cut edge portion of the thermal head such that the angle of separation .theta. of the thermal transfer ribbon 8 will be reduced. This makes the printed letters obscure.
If the thermal head is subjected to the cutting operation after the protective film 6 has been formed as shown in FIG. 6(b), the upper edge 200 of the thermal head will be formed with a very sharply angled corner. When such a sharp edge 200 is brought into contact with the thermal transfer ribbon 8 or a heat-sensitive sheet on slide movement of the thermal head, there may be created various problems such as separation of ink from the ribbon 8, darkened background of the heat-sensitive sheet and others. Thus, the upper sharp edge 200 of the thermal head or protective film 6 must be abraded to form a gentle curvature.